I can't update my iPhone

iPhone 6s Plus, iOS 15

If it is an iPhone 6S Plus you can update to iOS 15.7.7, you can check by reading this Apple article: iPhone models compatible with iOS 12 -> iOS 16.

iPhone models compatible with iOS 15.5 – Apple Support (UK)

You need about 6 GB device storage and good Service and/or Wi-Fi.

If they are On, turn Off Hotspot & VPN.

Charge the battery > 70%.

Go to Settings -> General -> Software Update -> Download and Install.

See the following Apple articles:

Update your iPhone, iPad, or iPod touch

Update your iPhone, iPad, or iPod touch - Apple Support (IN)

If your iPhone, iPad or iPod touch won’t update

If your iPhone, iPad, or iPod touch won't update - Apple Support

You can do the update via PC, see this Apple article: Update software on iPhone, iPad or iPod touch in iTunes on PC

Update software on iPhone, iPad or iPod touch in iTunes on PC – Apple Support (UK)

If you have a Mac see this Apple article: Update your iPhone, iPad, or iPod touch with the Finder

Update your iPhone, iPad, or iPod touch with the Finder - Apple Support

See:

sunnyas1 wrote: "...its restarting again and again by itself..."

Does your iPhone 6s still have its original battery?

And what's the Maximum Capacity % here?...

Settings > Battery > Battery Health

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sunnyas1 wrote: "No i changed my battery since an year ago. And its on 100%"

The maximum capacity shouldn't still be 100% after a year. ...Was the battery changed by a qualified technician using a genuine iPhone battery?...

About genuine iPhone batteries

Repair Provider Verification - Official Apple Support

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What did you try to fix the problem? Did you try and restart the device to see if the issue persists or gets solved this way? Does your question cover the iPhone 6s plus with an installation of iOs 15? It seems there's no new version of OS available and I wonder if this device should actually be supporting iOs 15 in the first place.